It is well known in the metal coating industry that there is a continuing need for coatings used for paint bonding and corrosion resistance. This is especially true where the metal is steel, aluminum, magnesium, aluminum alloys and zinc surfaced articles including galvanized iron or steel, where such coatings are required in order to protect the articles against deterioration.
In the past many such coatings have been suggested and used such as phosphate, zincate and anodized films but, in general, these coatings have left much to be desired because of various deficiencies including complex and often difficult operating procedures and high costs.
Chromate conversion coatings have been used with varying degrees of success. The effective protective ingredient in these coatings seems to be chromium in the hexavalent state. It has long been recognized that it would be desirable to combine in a single coating composition an organic resinous film forming component which is water insoluble and chromium in a hexavalent state. In particular, it would be desirable to prepare an emulsion coating composition containing the resinous organic film-forming component in the discontinuous phase and water in the continuous phase, together with chromium in a hexavalent state. If this could be done, latex emulsion paints such as, for example, acrylic emulsion paints, could be applied to metal substrates and a single coating would give a combined beneficial effect due to the presence of the chromium in the hexavalent state as well as the film-forming protective properties of the organic resin. Unfortunately, attempts to accomplish this result have not been successful primarily for the reason that the addition of the chromium in a hexavalent state by the usual means employing potassium dichromate, sodium dichromate, ammonium dichromate, (and their respective chromates) or chromic acid causes coagulation of the emulsion, produces large amounts of sediment, and destroys its capability of being applied as a uniform coating.